Spatial light modulators (SLM) have been used in projection display systems to increase image resolution and display brightness. For example, a Digital Micromirror Device (DMD) array, as described in U.S. Pat. Nos. 5,535,047 and 6,232,936, was used for two-dimensional image projection devices. According to this teaching, each micromirror of the DMD array has single-degree-of-freedom, rotation about an axis, and works as a simple optical switch. Since the DMD array is merely an array of optical switches, the direction of light is limited. As shown in FIG. 1, the DMD array has only two positions; one is the “on” position and the other is the “off” position. When the DMD array is applied to a two-dimensional image projection device, such as projectors and projection televisions, simple “on-off” behavior limits its light efficiency and becomes the main reason for its high power consumption. According to the prior art, the DMD array uses at most fifty percent (50%) of incident light because it only has “on” or “off” positions. In that regard, the light is dumped when the mirror is at it “off” position. In order to improve brightness and power efficiency of two-dimensional image projection system, most of the reflected light should be projected onto the screen.
In the prior art, a lens comprising a micromirror array was proposed by Boyd and G. Cho in their paper entitled, “Fast-response Variable Focusing Micromirror Array Lens”, Proc. SPIE, Vol. 5055, 278–286 (2003). Although micromirror array lens is capable of changing its focal length in a very sort amount of time, the micromirror array lens is not capable of being used in two-dimensional display devices because its optical axis is fixed. Moreover, since the micromirror array lens is a reflective type lens with a fixed optical axis, additional optical components, such as beam splitter, are required. The additional optical components deteriorate the light efficiency, and increase complexity and cost of the optical system.
Thus, there is a practical need for a two-dimensional image projection system that incorporates the advanced focusing capabilities of micromirror array lenses to improve brightness and power efficiency over existing projection systems. It is desired that such system be easy to manufacture and capable of being used with existing two-dimensional projections systems and devices.